Computational Modeling of Interdiffusion Microstructures

Ohio State University Research Foundation -Do Not Use, Columbus OH

Investigators

Abstract

TECHNICAL: In recent years, both experimental characterization and computer simulations have revealed many new phenomena that are not yet fully explained by existing theories and models. A list of examples include demixing of interdiffusion microstructures, 'horns' in multiphase diffusion paths, and special points on phase diagram that seem to act as 'strange attractors' for diffusion paths. The motion of markers, Type-0 boundaries, and precipitates caused by the Kirkendall effect are topics that could be added to the list. In addition, there are topics that have received little or no attention, for example, the effect of deviations from local equilibrium conditions on diffusion paths, the effect of precipitate coarsening on interdiffusion, and the phenomenon of concentration-gradient induced rafting. Uncertainty about why and when these phenomena occur has hindered attempts to answer questions of importance to designers. In this renewal project, the analytical, computational and experimental efforts of an on-going program will be extended to predict interdiffusion microstructures in multi-component, multi-phase diffusion couples. The goal will be to define the scientific principles underlying multi-component diffusion in multi-phase systems and then to apply them to predicting microstructures using tools like diffusion paths and phase diagrams. To model phenomena that are in local equilibrium and independent of precipitate morphology DICTRA software will be used. However to account for the effects of precipitate morphology and supersaturation (non-equilibrium) and to model interdiffusion microstructures containing more than one matrix phase, the phase field models already developed by PIs will be extended. The extended models will be able (a) to include nucleation of precipitates under the influence of a pre-existing microstructure and concentration gradient, (b) to develop a new approach based on the Kim-Suzuki model to overcome the intrinsic length scale limit of quantitative phase field modeling, and (c) to account for three-phase equilibrium. The analytical and modeling capabilities developed will be validated against carefully designed experiments and applied to develop fundamental understanding of the aforementioned phenomena. Also they will be applied to extend the new paradigm of internal oxidation that was proposed in the current program. NON-TECHNICAL: The ability to predict and understand interdiffusion microstructures has many applications in the broad field of high temperature processing and materials and will be the foundation for future advances in process and alloy design. This program draws from scientific and technological thrusts to better understand multi-component diffusion and to design more robust high-temperature coatings and alloys. To make the results generally available, PIs plan to place their computational methods and experimental and simulation findings on an open web site for use by the academic and industrial communities. In particular, a free down-loadable two-dimensional (2D) phase field open source code for interdiffusion in diffusion couples of arbitrary microstructures for Ni-Al-Cr system will be provided. In the meantime, PIs will integrate the knowledge, modeling methods and software programs into undergraduate and graduate curricula at OSU. In particular, a new graduate course: Analytical and Computational Methods of Diffusion in Multi-Component and Multi-Phase Systems will be developed and a new module on applying DICTRA and phase field software programs to solve multi-component and multi-phase diffusion problems will be introduced to the existing undergraduate course on materials modeling (MSE533).